[libcxx-commits] [libcxx] [libc++] Optimize filebuf::seekoff (PR #211788)

[Louis Dionne via libcxx-commits]

================
@@ -995,24 +995,80 @@ basic_filebuf<_CharT, _Traits>::seekoff(off_type __off, ios_base::seekdir __way,
     std::__throw_bad_cast();
 
   int __width = __cv_->encoding();
-  if (__file_ == nullptr || (__width <= 0 && __off != 0) || sync())
+  if (__file_ == nullptr || (__width <= 0 && __off != 0))
     return pos_type(off_type(-1));
-  // __width > 0 || __off == 0
-  int __whence;
+
+  auto __byte_off = __width <= 0 ? 0 : __width * __off;
+
+  int __whence = 0;
   switch (__way) {
-  case ios_base::beg:
-    __whence = SEEK_SET;
-    break;
-  case ios_base::cur:
+  case ios::beg: {
+    if (__off < 0)
+      return pos_type(off_type(-1));
+
+    pos_type __success_pos = __off;
+    __success_pos.state(__st_);
+
+    if (__always_noconv_ && __cm_ & ios::in) {
+      off_type __file_pos    = __ftell(__file_);
+      auto __buffer_base_pos = __file_pos - (this->egptr() - this->eback());
+
+      // If the new position is within the buffer we can just move the current read pointer
+      if (__off >= __buffer_base_pos && __off <= __file_pos) {
+        this->setg(this->eback(), this->eback() + (__off - __buffer_base_pos), this->egptr());
+        return __success_pos;
+      } else {
+        // Otherwise drop the buffer and seek to the requested position
+        this->setg(nullptr, nullptr, nullptr);
+        __cm_ = 0;
+        if (__fseek(__file_, __byte_off, SEEK_SET))
+          return pos_type(off_type(-1));
+        return __success_pos;
+      }
+    }
----------------
ldionne wrote:

I think I'd add an `else` here. There is a lot of control flow and it's not evident that the `if (sync())` below is mutually exclusive with the `__always_noconv` check.

https://github.com/llvm/llvm-project/pull/211788